Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called "LNG") for transport to market.
One of the distinguishing features of a LNG plant is the large capital investment required for the plant. The equipment used to liquefy natural gas is generally quite expensive. The liquefaction plant is made up of several basic systems, including gas treatment to remove impurities, liquefaction, refrigeration, power facilities, and storage and ship loading facilities. The plant's refrigeration systems can account for up to 30 percent of the cost.
LNG refrigeration systems are expensive because so much refrigeration is needed to liquefy natural gas. A typical natural gas stream enters a LNG plant at pressures from about 4,830 kPa (700 psia) to about 7,600 kPa (1,100 psia) and temperatures from about 20.degree. C. (68.degree. F.) to about 40.degree. C. (104.degree. F.). Natural gas, which is predominantly methane, cannot be liquefied by simply increasing the pressure, as is the case with heavier hydrocarbons used for energy purposes. The critical temperature of methane is -82.5.degree. C. (-116.5.degree. F.). This means that methane can only be liquefied below that temperature regardless of the pressure applied. Since natural gas is a mixture of gases, it liquefies over a range of temperatures. The critical temperature of natural gas is typically between about -85.degree. C. (-121.degree. F.) and -62.degree. C. (-80.degree. F.). Natural gas compositions at atmospheric pressure will typically liquefy in the temperature range between about -165.degree. C. (-265.degree. F.) and -155.degree. C. (-247.degree. F.). Since refrigeration equipment represents such a significant part of the LNG facility cost, considerable effort has been made to reduce refrigeration costs.
Although many refrigeration cycles have been used to liquefy natural gas, the three types most commonly used in LNG plants today are: (1) "cascade cycle" which uses multiple single component refrigerants in heat exchangers arranged progressively to reduce the temperature of the gas to a liquefaction temperature, (2) "expander cycle" which expands gas from a high pressure to a low pressure with a corresponding reduction in temperature, and (3) "multi-component refrigeration cycle" which uses a multi-component refrigerant in specially designed exchangers. Most natural gas liquefaction cycles use variations or combinations of these three basic types.
A multi-component refrigerant system involves the circulation of a multi-component refrigeration stream, usually after precooling to about -35.degree. C. (-31.degree. F.) with propane. A typical multi-component system will comprise methane, ethane, propane, and optionally other light components. Without propane precooling, heavier components such as butanes and pentanes may be included in the multi-component refrigerant. The nature of the multi-component refrigerant cycle is such that the heat exchangers in the process must routinely handle the flow of a two-phase refrigerant. Multi-component refrigerants exhibit the desirable property of condensing over a range of temperatures, which allows the design of heat exchange systems that can be thermodynamically more efficient than pure component refrigerant systems.
One proposal for reducing refrigeration costs is to transport liquefied natural gas at temperatures above -112.degree. C. (-170.degree. F.) and at pressures sufficient for the liquid to be at or below its bubble point temperature. For most natural gas compositions, the pressure of the PLNG ranges between about 1,380 kPa (200 psia) and about 4,500 kPa (650 psia). This pressurized liquid natural gas is referred to as PLNG to distinguish it from LNG which is at or near atmospheric pressure and at a temperature of about -160.degree. C. PLNG requires significantly less refrigeration since PLNG can be more than 50.degree. C. warmer than conventional LNG at atmospheric pressure.
A need exists for an improved closed-cycle refrigeration system using a multi-component refrigerant for liquefaction of natural gas to produce PLNG.